1. Field of the Invention
The present invention relates methods of and apparatus for determining wood grain orientation for use, for example, in lumber grading, defect inspection and quality control.
2. Description of the Related Art
The detection of wood grain orientation is normally carried out by human observation and decision. However, modem forest industry requires scanning technology to measure wood grain orientation automatically, reliably and precisely.
Wood is generally regarded as an orthogonal anisotropic material. The anatomic anisotropy of wood cells gives rise to a grain structure with a grain orientation determined by longitudinal axes of the wood cells. The interaction of wood materials with various types of radiation brings about an anisotropic distribution of the radiation, which can be used as an indication of the wood grain orientation.
Based on the anisotropic distribution of the dielectric constant of wood materials, a slope-of-grain indicator (SGI) has been developed to determine the grain orientation of softwood and hardwood species. For this purpose, a transducer with capacitor plates is mounted in close proximity to a wood surface and rotated for dielectric constant scanning and sensing. When the direction of an electric field applied to the capacitor plates is parallel to the wood grain orientation, the transducer capacitance, which is proportional to the wood dielectric constant, reaches its maximum. The resolution of the SGI depends on the transducer diameter (xc2xe or 1xe2x85x9c inch). The speed could be up to 3600 rpm. The precision might approach xc2x11xc2x0, provided the transducer-to-wood distance is maintained within a prescribed limit (xe2x85x9 or xc2xc inch).
However, when SGI is employed, the measurement of the grain orientation is not independent of the gradients of wood moisture and density. SGI is also sensitive to the transducer-to-wood distance, unless the distance is maintained within the above prescribed limit, resulting in close-contact measurement with less flexibility.
Based on the attenuation, phase shift and depolarization of a polarized microwave transmitted through wood, a microwave transmission method has been suggested to probe the amplitude, phase and polarization of a microwave field and to measure the moisture, density and grain orientation of wood materials. Approximately, the attenuation reflects moisture, the phase shift reflects both moisture and density, and the depolarization reflects grain orientation. This method was improved to cope with the dependence of depolarization on moisture and density, which makes grain orientation prediction rather complicated. The experimental data show that the method can measure the grain orientation reliably up to 30xc2x0 at a speed up to 40 kHz and a resolution about 2 cm.
It is, however, a disadvantage of the microwave transmission method that calibrations are complicated because of a considerable interaction between the parameters. The grain orientation can be deduced reliably only when the specimen thickness and moisture are great enough to introduce sufficient dielectric anisotropy to appreciably depolarize the incident wave. The apparatus and techniques are very complex, although the principles and experiments show the method to be feasible for on-line lumber grading by recording moisture, density and grain orientation in real time.
Instead of employing human observation and decision, a scanning system consisting of a camera and a computer for pattern recognition has been used to detect wood defects. This system can be used to determine wood grain orientation based on wood splits, seasoning checks, and mainly demarcations between earlywood and latewood. The precision and speed depend on the algorithms designed for pattern recognition.
However, this scanning system for pattern recognition, which is time consuming, may not give correct grain orientation because the demarcations between earlywood and latewood, especially in the case of flatsawn surfaces, are not reliable indicators of grain orientation.
Several scanning systems based on the interaction of wood materials with laser light have been proposed to detect wood grain orientation. When a laser light strikes a wood surface, the laser light is split into two parts: a reflected light bouncing off the wood surface and a transmitted light propagated in the wood material, which is an anisotropic absorbing medium. Light propagated across wood cell walls will be attenuated more than light propagated along wood cell lumens, where the attenuation constant of air or water is almost zero. In the process of propagation, some of the transmitted light will emerge from the wood surface, forming an elongate laser spot having its major axis along the grain orientation as a result of the highly directional propagation beneath the wood surface. Therefore, the light on the wood surface is a mixture of emergent laser light, reflected laser light, and non-laser illumination.
The shape of the reflected laser spot is affected by wood surface qualities (planed, sanded, or sawn). Generally, a roughsawn surface will cause more jagged edges on the reflected laser spot than other wood surfaces. The elongate emergent laser spot (caused by wood cell structure) is mixed with the jagged reflected laser spot (caused by wood surface roughness), bringing about the inaccurate information of wood grain orientation.
The shape of the emergent laser spot is affected by wood species. Because the cells of hardwood species are thicker and shorter than those of softwood species, the transmitted laser light will be attenuated more and propagated less in hardwood materials, resulting in a smaller emergent laser spot appearing on the hardwood surface. For the same incident light, the length of the emergent laser spot on a hardwood surface is less than half of that on a softwood surface. The aspect ratio of the emergent laser spot on a hardwood surface could be as low as 1.2:1, while that on a softwood surface is around 2:1. The shape of the emergent laser spot is also affected by wood abnormalities, such as pores in hardwood species and rays in radial sections, resulting in the distortion of the emergent laser spot.
For precision measurement, softwood species and planed wood materials are preferable. The small aspect ratio of the emergent laser spot on a hardwood surface, or jagged edges of the reflected laser spot on a roughsawn surface, will impair the anisotropic distribution of the laser light and make it difficult to determine the grain orientation of hardwood species and roughsawn materials by means of laser scanning.
The present inventors have, however, found that wood grain orientation can be satisfactorily determined by means of laser scanning by taking into account the fact that the anisotropic distribution of laser light on a wood surface, which is an indication of the wood grain orientation, can be enhanced by employing a polarizer to separate the emergent laser light from the reflected laser light, and using phase demodulation to eliminate noise from the image.
According to the present invention, a method of determining the grain orientation of a piece of wood comprises the steps of projecting a beam of linearly polarized laser light onto a wood surface to produce an image comprising elliptically polarized emergent laser light and linearly polarized reflected laser light, in addition to non-laser illumination; employing a polarizer to suppress the reflected laser light and a filter to block the non-laser illumination; and capturing, digitizing and processing the image of the emergent laser light to extract the image orientation indicative of the grain orientation.
More Particularly, the Present Inventors Have Found That:
1. The emergent laser light can be separated from the reflected laser light.
When a linearly polarized laser light strikes a wood surface, the reflected laser light will keep its state of polarization for the most part, while the transmitted laser light will be gradually depolarized due to the dielectric anisotropy of wood cells. After passing through sufficient crystalline regions of wood cellulose, the laser light emerging from the wood surface will become elliptically polarized. By employing a polarizer, the emergent laser light, which carries grain orientation information and is mainly elliptically polarized, can be partially separated from the reflected laser light, which is sensitive to wood surface roughness and mostly linearly polarized.
2. The spot of the emergent laser light can be enlarged to its saturation size.
When the laser output is not very high ( less than 1 mW), on a softwood surface, the emergent laser spot is much dimmer but bigger than the reflected laser spot, while on a hardwood surface, the emergent laser spot is close to the reflected laser spot in both brightness and size. With an increase of the laser output (1xcx9c30 mW), the reflected laser spot is amplified proportionally, but the emergent laser spot is enlarged gradually to its saturation size. For hardwood species, a bigger emergent laser spot is needed to cope with spot distortion caused by wood abnormalities. By employing a polarizer, the amplified reflected laser spot can be suppressed and the enlarged emergent laser spot can be retained having its saturation size.
3. Laser image orientation can be extracted by one-dimensional phase demodulation.
First, the image of a laser spot or spot-matrix on a wood surface can be transformed into a one-dimensional periodic signal. For the laser spot, the transformation comprises the steps of establishing a polar coordinate system with the pole at the center of the laser spot, and scanning the light intensity of each ray as a function of polar angle. For the laser spot-matrix, the transformation comprises the steps of applying a circular operator to the image, scanning the orientation vector of each pixel of the image, and calculating a histogram of image orientation.
Second, by calculating the real and imaginary part of the second spectrum of the one-dimensional periodic signal, the initial phase, which indicates laser image orientation, can be demodulated from a jagged amplitude, which reflects laser image noise caused by surface roughness, wood abnormalities, and the orientation variation of the wood cells, which are sampled by the laser spot or spot-matrix.
Third, the greater the contrast of the periodic signal, the higher the precision of the phase demodulation. With an increase of the laser output, the signal contrast increases accordingly. Once the signal contrast reaches its maximum, any decrease of the signal contrast may be used as a feedback signal to adjust the laser output for maximizing the signal contrast and increasing the demodulation precision.
Based on the above findings, the present inventors have developed a laser scanning system for wood grain orientation determination. The system includes a polarizing prism for separating the emergent laser light from the reflected laser light, and a function generator for enlarging the emergent laser spot or maximizing the signal contrast, and may employ an algorithm of phase demodulation for signal transformation, phase demodulation, and contrast feedback. The system is capable of extracting wood grain orientation quickly and precisely from laser images with low signal-to-noise ratio, such as the images on the surfaces of hardwood species and roughsawn materials.